The possibility of identifying the geographical location of a mobile terminal or other node in a network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising and emergency calls. Different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, i.e. the FCC (Federal Communications Commission) E911 in the USA.
In many environments, the geographical position of a node can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Today, networks also often have the possibility to assist e.g. UEs (User Equipment) in order to enable the UEs to perform measurements at much lower receiver sensitivity level and improve GPS cold start or start up performance through so called A-GPS (Assisted-GPS) positioning. However, GPS, or A-GPS, receivers are not necessarily available in all wireless terminals. Furthermore, GPS is known to often fail in indoor environments and urban canyons, due to lack of satellite coverage. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3GPP (3rd Generation Partnership Project).
OTDOA Positioning
With OTDOA, a mobile terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each neighbor cell to be measured, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between the neighbor cell and a reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. An example of an OTDOA scenario is illustrated in FIG. 1. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In order to obtain an estimate of the geographical position, precise knowledge of the transmitter locations and transmit timing offset is needed. Position calculation can be conducted, for example, by a positioning server, such as the eSMLC (evolved Serving Mobile Location Center) in Long Term Evolution (LTE), or by a UE. The former approach corresponds to the UE-assisted positioning mode which is the only OTDOA mode standardized so far in 3GPP, whilst the latter corresponds to the UE-based positioning mode.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals have been introduced, namely so called Positioning Reference Signals (PRSs), which are dedicated for positioning. Further, low-interference positioning sub-frames have been specified in 3GPP.
The PRS are transmitted from one antenna port according to a pre-defined pattern, as described in 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation. A frequency shift, which is a function of PCI (Physical Cell Identity), can be applied to the specified PRS patterns to generate orthogonal patterns and thus enable an effective frequency reuse of six. The use of such a frequency shift makes it possible to significantly reduce neighbor cell interference on the measured PRS and thus improve positioning measurements. OTDOA assistance information and neighbor cell lists.
Since for OTDOA, positioning PRS signals from multiple distinct locations need to be measured, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when in time the measured signals are expected to arrive, and what the exact PRS pattern is, the UE would need to do signal search within a large window, which would have an impact on the time and accuracy of the measurements, as well as the UE complexity. To facilitate UE measurements, the network therefore transmits assistance data to the UE, which includes e.g. an NCL comprising the physical cell identities (PCIs) of neighbor cells, the number of consecutive downlink positioning subframes, PRS transmission bandwidth, etc. The NCL may also comprise the cell identity of the serving cell. The serving cell may thus be considered to be included in the term “neighboring cells”, when appropriate.
The neighbor cell lists are typically defined by the network and then signaled to UEs over LPP (LTE Positioning Protocol). The PCI of a cell maps to the PRS pattern. Hence, from the received assistance information, the UE should be able to deduce the sequence transmitted from each neighbor cell indicated in the assistance data, PRS pattern, etc. The lists may also provide additional information, such as the expected signal quality of signals from the corresponding cells. Such information may be explicit, e.g. such as when a quality metric is signaled together with each neighbor cell identity, or it may be implicit, e.g., such as when the neighbor cells are just arranged in a certain order in the list according to some criteria, and the UE is able to correctly interpret the order.
It has been agreed in 3GPP that the maximum size of positioning neighbor cell lists is 24 cells per carrier. In multi-carrier system, which comprises more than one carrier, generally known as a component carrier, a neighbor cell list may be signaled for each component carrier, i.e. 24 cells per component carrier.
There have been intensive discussions in 3GPP regarding what maximum number of cells to use in the positioning neighbor cell list. From the UE-side perspective, large lists lead to increased UE complexity, require larger buffers, longer measurement time until the position fix, etc., as compared to smaller lists. However, to ensure performance of measurements on the required number of neighbor cells, and the required positioning accuracy, the neighbor cell lists need to be sufficiently large, also to take into account the dynamic nature of the traffic and radio environment in general. Because of the latter, it is a very challenging task when configuring the network to design reasonably short, but yet reliable UE-specific neighbor cell lists. This becomes even more complicated, for example, due to the flexibility of LTE deployment scenarios in terms of synchronization (LTE can be synchronous or asynchronous) and supported duplex modes (FDD, TDD or half duplex).
Therefore, the maximum size of positioning neighbor cell lists agreed for LTE, i.e. 24 cells per carrier, is smaller than for corresponding lists in, e.g., UTRAN, with 32 neighbor cells per carrier, or CDMA2000, with 40 neighbor cells per carrier. This makes implementation of positioning network solutions more complicated and less compatible, and may require a significant network re-planning, which is typically very costly for operators. Also, UE design and algorithms may become less consistent among different systems and more difficult to adapt from one system to another, although the recent trend is that a UE supports multiple radio access technologies.
The different requirements of the number of identified neighbor cells needed in order to e.g. achieve measurements of a required quality, and the conflicting interests regarding the size of neighbor cell lists described above, have been identified as a problem, since UEs may not have access to adequate information on a sufficient number of neighbor cells for performing certain measurements.